The present invention relates to air conditioning systems of either the heating or cooling type wherein air from a room or building is drawn into the unit and passed over heating or cooling coils prior to being recirculated back into the room or building. The present disclosure is primarily concerned with air conditioning systems of the cooling type, but the invention is equally applicable to heating systems, or heating/cooling apparatus such as heat pumps.
In many central air conditioning systems presently in use, a thermostat deployed at some point in the building senses the temperature of the ambient air and if it is higher than the comfort setting which has been selected, activates the air conditioning unit. Such a unit normally comprises a compressor, condenser, and evaporator connecting with each other in a closed refrigerant system. The gaseous refrigerant is delivered from the compressor to the condenser coil where it gives up heat and then is passed through an expansion valve to the evaporator coil where it absorbs heat from the circulating air which is passed thereover by the evaporator fan. When the thermostat senses that the ambient air has been cooled to the desired level, the compressor, evaporator fan and condenser fan are turned off until the ambient temperature has again reached the level where further cooling is necessary.
Although the compressor has been deactivated, the evaporator coils remain cool because of the fact that the refrigerant is vaporized at a low temperature. Normally, no utilization is made of the cooling capacity of the evaporator coil, however, because the evaporator fan is turned off with the compressor. This results in inefficient utilization of the energy required to compress the refrigerant.
In almost all window air-conditioners, the evaporator fan runs constantly and the compressor is cycled on and off depending on the temperature of the ambient air sensed by the thermostat. Although circulating air continues to pass around the evaporator coil even after the compressor has been turned off so that as much heat is possible is imparted to the coil, the air will continue to circulate even after the temperature of the evaporator coil has attained the temperature of the air. This often results in an unpleasant odor being imparted to the circulating air produced by certain microorganisms and other contaminants such as nicotine which collect on the evaporator coil and in the drip pan of the air conditioner. U.S. Pat. No. 3,762,178 discusses this problem and attempts to solve it by delaying the start of the evaporator fan until the evaporator coil has reached a temperature where dew forms thereon. Although such a technique will tend to eliminate the unpleasant odor, the evaporator fan is still cycled off with the compressor so that the full cooling capacity of the evaporator coil is not utilized.
In certain situations, the compressor runs constantly and the customary thermostat controlled cycling does not occur. Although continuous operation of the system may be necessary for proper cooling and ventilation, prolonged running of the compressor will be deleterious from the standpoint of its operating life and may necessitate that a larger capacity motor be used than that which would normally be required for the compressor in order to accommodate the increased load.
Although prior art systems which employ a timer cycled compressor are known, the same results as those achieved by the present invention are not realized. In U.S. Pat. No. 3,545,218, a timer is utilized to alternately switch the compressor on and off but the duration of the "on" period relative to the "off" period varies depending on the ambient temperature sensed by the thermostat. Such an arrangement does not permit the duration of the relative intervals to be absolutely controlled and results in the compressor being restarted before maximum cooling is achieved by the deactivated system. In U.S. Pat. No. 2,969,652, a time clock is utilized to energize the compressor at a selected time of day, for example in the morning, and de-energize the compressor at a later time, for example at night. The evaporator fan is also placed under the control of the clock, however, and ceases to run when the compressor is de-energized.